It probably wasn't written too well because it seems a lot of people missed it (and I missed it the first time I saw the episode either), but Star didn't deliberately crash the party out of spite or anything.
Brittney says Star can go "when pigs fly;" Star is retarded enough not to understand the expression or that Brittney obviously hates her and takes it to mean that Brittney is asking for a flying pig, which she makes. After arriving to the party and realizing how pissed Brittney is Star says to Marco something about how she's realizing they weren't actually invited.
Star and Marco can't leave because the bus is stuck in traffic, though. The minute it isn't, the two of them actually try to step off, only to be forced back onto the bus by Ludo and his gang and a monster fight ensues and the bus crashes etc.
tl;dr: Star didn't realize she was crashing it and once she did, she tried to leave